Resistive random access memory structure with tri-layer resistive stack

ABSTRACT

An RRAM includes a resistive layer including a dielectric layer and surplus oxygen ions or nitrogen ions from a treatment on the dielectric layer after the dielectric layer is formed. When the RRAM is applied with a voltage, the oxygen ions or nitrogen ions occupy vacancies in the dielectric layer to increase resistance of the resistive layer. When the RRAM is applied with another voltage, the oxygen ions or nitrogen ions are removed from the vacancies to lower the resistance of the resistive layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a memory, and particularly to an RRAMstructure and a method of making the same.

2. Description of the Prior Art

Resistive random access memory (RRAM) is a new memory structure createdin the semi-conductive field. An RRAM stores data by using the variableresistance characteristic of a dielectric layer interposed between twoelectrodes. Such dielectric layer, as a resistive layer, is normallyinsulating but can be made to be conductive through a filament orconduction path formed after application of a sufficiently high voltage,i.e. through a forming process. The conduction path formation can arisefrom different mechanisms, including defects, metal migration, etc. Oncethe filament is formed, it may be reset (i.e. broken, resulting in highresistance) or set (i.e. re-formed, resulting in lower resistance) by anappropriately applied voltage.

FIGS. 1 and 2 show a schematic cross-sectional view of a conventionalRRAM at a low resistance status and a high resistance status,respectively. As shown in FIGS. 1 and 2, an RRAM 10 includes a bottomelectrode 12, a resistive layer 14 and a top electrode 20. A formingprocess has been performed on the RRAM 10, during which a graduallyincreasing voltage is applied to the top electrode 20 and the bottomelectrode 12, and then the current flowing through the bottom electrode12, the resistive layer 14 and the top electrode 20 is raised to acompliance current. In this way, the quality of the resistive layer 14becomes non-uniform and the upper part of the resistive layer 14 willhave fewer defects than the lower part of the resistive layer 14. Whenthe RRAM is working upon application of a voltage, current filament 22occurs in the resistive layer 14. As shown in FIG. 1, the currentfilament 22 distributes from the bottom electrode 12 to the topelectrode 20 to make the RRAM 10 have a low resistance. As shown in FIG.2, the current filament 22 is broken at the upper portion of theresistive layer 14 to make the RRAM 10 have a high resistance.

The forming process mentioned above is highly complicated andtime-consuming. Furthermore, the forming process is performedelectrically through a random way, in which breakdown is easily out ofcontrol, resulting in low yield.

Therefore, there is still a need for a novel RRAM which can be madeeasily and has an excellent performance.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an RRAM structure notrequiring a time-consuming forming process and the resistance variationis easily controlled.

Another object of the present invention is to provide a method of makingthe RRAM structure as mentioned above.

The RRAM structure according to an embodiment of the present inventioncomprises a bottom electrode, a top electrode and a resistive layer. Theresistive layer is sandwiched between the bottom electrode and the topelectrode. The resistive layer comprises a dielectric layer formed of adielectric material with a plurality of vacancies as being structuraldefects. The resistive layer further comprises oxygen ions or nitrogenions from a treatment on the dielectric layer after the dielectric layeris formed. When the RRAM is applied with a voltage, the oxygen ions ornitrogen ions occupy the vacancies to increase resistance of theresistive layer. When the RRAM is applied with another voltage ofopposite polarity, the oxygen ions or nitrogen ions are removed from thevacancies to lower the resistance of the resistive layer.

The method of forming an RRAM structure according to an embodiment ofthe present invention comprises steps as follows. A bottom electrode isformed. A dielectric layer is formed on the bottom electrode. An oxygentreatment or a nitrogen treatment is performed on the dielectric layerto introduce oxygen or nitrogen ions into the dielectric layer. A topelectrode is performed on the dielectric layer.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show a schematic cross-sectional view of a conventionalRRAM structure at a low resistance status and a high resistance status,respectively;

FIG. 3 shows a schematic cross-sectional view of an RRAM structureaccording to an embodiment of the present invention;

FIGS. 4 and 5 schematically show a dielectric layer of an RRAM structureaccording to an embodiment of the present invention at a low resistancestatus and a high resistance status, respectively;

FIG. 6 shows an RRAM structure according to an embodiment of the presentinvention;

FIG. 7 illustrates a method of forming an RRAM structure according to anembodiment of the present invention;

FIG. 8 shows results of concentration distribution of oxygen determinedby secondary ion mass spectrometry (SIMS);

FIG. 9 illustrates a method of forming an RRAM structure according toanother embodiment of the present invention; and

FIG. 10 shows a plot of current versus bias resulted from a measurementof an RRAM according to an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 3 shows a schematic cross-sectional view of an RRAM structureaccording to an embodiment of the present invention. An RRAM 30 includesa bottom electrode 32, a top electrode 36, and a resistive layer 34interposed or sandwiched between the bottom electrode 32 and the topelectrode 36. The resistive layer 34 includes a dielectric layer formedof a dielectric material with a plurality of vacancies as beingstructural defects. The resistive layer 34 further comprises oxygen ionsor nitrogen ions from a treatment after the dielectric layer is formed.

The structural defects commonly occur when a material is formed in bulk.In conventional RRAM, such structural defects are utilized to formcurrent filaments for variation of resistance upon application of avoltage after the RRAM is completely formed through a forming process.In the present invention, additional oxygen or nitrogen ions are furtherplaced into the dielectric material. When a positive voltage, forexample, 2 to 5 volts incase the oxide layer is titanium oxide with athickness of about 30 nm, is applied to the RRAM, the oxygen or nitrogenions occupy vacancies (the structural defects of the dielectricmaterial), resulting in a high resistance, and when a negative voltage,for example −2 to −5 volts, is applied to the RRAM, the oxygen ornitrogen ions are removed form the vacancies, resulting in a lowerresistance.

The bottom electrode and the top electrode of the present invention eachmay be one usually used in conventional RRAM, for example metal nitride(such as TiN), Pt, AlCu, Au, Ti, Ta, TaN, W, WN, Cu and the like, butnot limited thereto.

The dielectric layer may comprise for example oxide material, such as anoxide compound of metal, such as an oxide compound of nickel, titanium,hafnium, aluminum, zirconium, zinc, tungsten, aluminum, tantalum,molybdenum, copper or the like, or an oxide compound of silicon, ornitride material such as silicon nitride. Theoretically, any oxide insolid state with vacancies can serve as the dielectric layer in thepresent invention.

The thickness of the dielectric layer depends on the desired design ofRRAM, and is preferably more than 5 nm, and more preferably 30 to 60 nm.The oxygen ion may be O²⁻. The nitrogen ion may be N³⁻. The amount ofoxygen or nitrogen ions additionally introduced into the dielectriclayer may be for example in a range from 20% to 40% of the amount of theoxygen or nitrogen of the dielectric material. It may be preferred thatthe amount of additional oxygen or nitrogen ions is greater than theamount of vacancies of the dielectric material.

In one embodiment, the dielectric layer may be a TiO_(x) layer. Thesymbol, x, may be preferably a number in a range from 1.5 to 2. TheTiO_(x) layer, which may be amorphous or crystalline, has vacancies, asa generally formed dielectric layer does. The introduced oxygen ornitrogen ions occupy the vacancies.

FIGS. 4 and 5 schematically show a resistive layer of an RRAM accordingto an embodiment of the present invention at a low resistance status anda high resistance status, respectively. The resistive layer 38 comprisesa dielectric material 40 with a plurality of vacancies 42 and further aplurality of oxygen or nitrogen ions 44. In FIG. 4, the vacancies arenot occupied, and electrons can flow via the vacancies, and thus theRRAM has a low resistance. In FIG. 5, the vacancies 42 are occupied bythe ions 44, and electrons can not flow via the vacancies 42, and thusthe RRAM has a high resistance.

The dielectric layer may be a single layer or a multi-layer. When it isa multi-layer, it is preferred that the additional oxygen or nitrogenions are placed in the layer beneath the top layer. The material of eachlayer of the multi-layer may be the same or different. FIG. 6 shows anembodiment in which the dielectric layer is formed of three layers, 46,48, and 50. The dielectric layer 46 and the dielectric layer 50 arethinner than the dielectric layer 48. For example, the thickness of thedielectric layers 46, 48, 50 may be less than 10 nm, between 40 and 10nm, and less than 10 nm, respectively. The dielectric layer 46 may serveas a barrier layer to block the diffusion of the oxygen or nitrogen ionsto the bottom electrode 32, thereby preventing the oxygen or nitrogenions from reacting with the bottom electrode. The dielectric layer 50can serve as a sacrificial layer or a screen layer to protect thedielectric layer 48 from being damaged by the impact of the ionimplants, alleviate channel effect and also serve as a barrier layer toprevent the oxygen or nitrogen ions from reacting with the topelectrode.

The RRAM structure according to the present invention may be madethrough steps as follows. A bottom electrode is formed by, for example,a sputtering process or an atomic layer deposition (ALD) process. Adielectric layer is formed on the bottom electrode by, for example,physical vapor deposition (PVD), chemical vapor deposition (CVD)process, atomic layer deposition (ALD), or metal sputtering followed byan oxidation, such as usually formed in a conventional technique.Thereafter, an oxygen treatment or a nitrogen treatment is furtherperformed on the dielectric layer to introduce oxygen or nitrogen ionsinto the dielectric layer. Thereafter, a top electrode is formed on thedielectric layer, which may be performed in the same way as the bottomlayer.

The oxygen or nitrogen ions may be introduced into the dielectric layerthrough a treatment on the dielectric layer. Preferably, when thedielectric layer is formed of oxide material to be an oxide layer,additional oxygen ions are introduced into the oxide layer, and when thedielectric layer is formed of nitride material to be a nitride layer,additional nitrogen ions are introduced into the nitride layer. Theoxygen treatment or the nitrogen treatment may be an ion implantationprocess or a plasma drive-in treatment.

The ion implantation is preferably performed to implant the oxygen ornitrogen ions into the dielectric layer at the middle height of thedielectric layer, particularly when the dielectric layer of the RRAM issingle-layered, for preventing the implanted ions from reacting with thetop or bottom electrode, but not limited thereto. FIG. 7 shows after thedielectric layers 46, 48, and 50 are formed, an ion implantation 52 isperformed to implant oxygen or nitrogen ions 54 through the sacrificialdielectric layer 50 and into the dielectric layer 48, preferably at ahalf depth of the dielectric layer 48. For example, when the dielectriclayer 48 is a titanium oxide layer with a thickness of about 15 nm, theoxygen ion implantation may be performed with energy of 10 to 25 KeV anda dose of 5×10¹⁵ to 5×10¹⁶ atoms/cm², using for example O₂ or CO as anion source. The implanted ions may distribute in the dielectric layer asa Gaussian distribution. Thermal treatment may be not required after theimplantation.

FIG. 8 shows a result of a concentration distribution of the oxygen,determined by SIMS, of the three oxide layers of the RRAM according tothe embodiment of the present invention as referred to FIG. 7, in whichthe three dielectric layers 50, 48, 46 are titanium oxide and labeled as1, 2, and 3, respectively. A result of a concentration distribution ofthe oxygen of three titanium oxide layers without additional oxygenintroduction is also shown for purpose of comparison. The oxide layer 2(the dielectric layer 48) has the highest oxygen content among the threeoxide layers. The oxide layer 1 (the dielectric layer 50) may containadditional oxygen ions. The oxide layer 3 (the dielectric layer 46)contains less additional oxygen ions.

The plasma drive-in treatment is preferably performed to place theoxygen or nitrogen ions into the dielectric layer. FIG. 9 shows that,after the dielectric layers 46, 48 are formed, a plasma drive-intreatment 56 is performed to dispose a layer of oxygen or nitrogen ions58 into the dielectric layer 48. Then, a thermal treatment may beoptionally performed to drive the oxygen or nitrogen ions into thedeeper portion of the dielectric layer 48. A buffer or oxygenblocking/barrier layer on the top of the dielectric layer 48 may beutilized in this embodiment.

FIG. 10 shows a plot of current versus bias resulted from a measurementof an RRAM according to an embodiment of the present invention. Thetested sample is an RRAM having a resistive layer structuringTiN/TiO_(1.5)/TiN in thickness of 10 nm/50 nm/10 nm and oxygen ionsimplanted within the TiO_(1.5) using an energy of 15 KeV and a dose of5×10¹⁵ atoms/cm². It shows a low program current density 28 mA/cm²(area: 50 μm×50 μm, current: 7 μA) and a low erase current density 4mA/cm² (area: 50 μm×50 μm, current: 1 μA). Furthermore, the retentiontime of the sample is determined to pass 10⁴ seconds at 125° C. Theuniformity is better, since the yield is substantially 100% as comparedto non-implanted conventional one of 48%. The RRAM of the presentinvention may also serve as a multi level cell (MLC), since it seemscapable of more than 4 resistance states. It is endurable since theendurance test achieves 10⁶ cycles.

The RRAM according to an embodiment of the present invention hasproperties of resistance switching based upon occupation of vacancies byoxygen or nitrogen ions. No vacancy movement or creation occurs duringthe occupation of vacancies, and thus no damage occurs to the dielectriclayer. Accordingly, the effective thickness of the dielectric layer willbe substantially not changed. As a result, a time-consuming formingprocess is not required and the resistance switching can be quick andeasily controlled.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention.

What is claimed is:
 1. A resistive random-access memory (RRAM)structure, comprising: a bottom electrode; a top electrode; and atri-layer resistive stack having a first surface and a second surfacewhich are parallel with each other and sandwiched between the bottomelectrode and the top electrode, wherein the tri-layer resistive stackcomprises a dielectric middle layer, a dielectric screen layer betweenthe dielectric middle layer and the top electrode, and a dielectricbarrier layer between the dielectric middle layer and the bottomelectrode, wherein said first surface is in direct contact with said topelectrode and said second surface is in direct contact with said bottomelectrode so that the dielectric barrier layer is disposed between thetop electrode and the bottom electrode, and said dielectric middle layerfurther comprises oxygen ions or nitrogen ions from a treatment on thetri-layer resistive stack after the tri-layer resistive stack is formed,said dielectric screen layer and said dielectric barrier layerrespectively block the diffusion of said oxygen ions or nitrogen ions tosaid top electrode and said bottom electrode.
 2. The RRAM structureaccording to claim 1, wherein the dielectric screen layer and thebarrier layer are thinner than the middle layer.
 3. The RRAM structureaccording to claim 1, wherein the dielectric screen layer and thedielectric barrier layer are made of a material that is different fromthe dielectric middle layer.
 4. The RRAM structure according to claim 1,wherein ions are disposed at a half depth of the dielectric middlelayer.
 5. The RRAM structure according to claim 4, wherein thedielectric middle layer is a nitride layer and the ions are nitrideions.
 6. The RRAM structure according to claim 1, wherein the dielectricbarrier layer and the dielectric screen layer are thinner than thedielectric middle layer.